Free-electron amplifier operation has been proposed based on the emission of electromagnetic radiation by accelerated high-energy electrons, acceleration typically being in a spatially periodic magnetic field whose direction is essentially transverse to electron velocity. A theoretical study of the emission of electromagnetic radiation by periodically accelerated electrons was made by H. Motz, "Applications of the Radiation from Fast Electron Beams", Journal of Applied Physics, Vol. 22 (1951), pp. 527-535, and experimental results were presented by H. Motz et al., "Experiments on Radiation by Fast Electron Beams", Journal of Applied Physics, Vol. 24 (1953), pp. 826-833.
More recently, amplification of infrared radiation by relativistic free electrons in a spatially periodic magnetic field was observed by L. R. Elias et al., "Observation of Stimulated Emission of Radiation by Relativistic Electrons in a Spatially Periodic Transverse Magnetic Field", Physical Review Letters, Vol. 36 (1976), pp. 717-720, and free-electron laser operation at a wavelength of 3.4 micrometers was reported by D. A. G. Deacon et al., "First Operation of a Free-Electron Laser", Physical Review Letters, Vol. 38 (1977), pp. 892-894. As shown, e.g., in U.S. Pat. No. 3,822,410, issued July 2, 1974 to J. M. J. Madey, free-electron laser apparatus typically includes components such as, in particular, a source of high-energy electrons, a source of a spatially periodic magnetic field, and two radiation reflecting elements of which one is essentially totally reflecting and the other is semitransparent to generated radiation.
Free-electron lasers are understood to be most promising for generating tunable far-infrared radiation. Accordingly, the following are considered relevant: R. Ulrich, "Far-Infrared Properties of Metallic Mesh and its Complementary Structure", Infrared Physics, Vol. 7, pp. 37-55 (1967), R. Ulrich, "Interference Filters for the Far Infrared", Applied Optics, Vol. 7, pp. 1987-1996 (1968), R. Ulrich et al., "Variable Metal Mesh Coupler for Far Infrared Lasers", Applied Optics, Vol. 9, pp. 2511-2516 (1970), C. O. Weiss, "Optically Pumped FIR-Laser with Variable Fabry-Perot Output Coupler", Applied Physics, Vol. 13, pp. 383-385 (1977), and E. D. Shaw et al., "Theoretical Considerations for FEL's in the Far Infrared", Free-Electron Generators of Coherent Radiation, Addison-Wesley, 1980, pp. 665-669.
A key feature of free-electron amplifier operation is amplification of electromagnetic radiation due to recoil of electrons during emission of electromagnetic radiation and attendant separation of the frequencies of emission and absorption. Amplification occurs at frequencies for which the transition rate for emission exceeds the transition rate for absorption, and the amplification factor is directly dependent on the duration of interaction between electromagnetic radiation and electrons. If a pulsed electron beam is used and if the speed of electrons is appreciably less than the speed of light, it may be that such duration of interaction is undesirably brief.